In the automotive industry, metal and plastic components are provided with multilayer coatings. These Coatings are intended to protect the substrates from corrosion, but are also intended to have a good decorative effect. Such coatings are generally multilayer coatings based on a primer and one or more subsequent layers, for example base lacquer layers, topcoat lacquer layers, which may then be provided with a clear lacquer as a subsequent coating.
Aqueous coating compositions having low proportions of organic solvents are known. Polyurethane resin dispersions stabilised with ionic groups in the aqueous phase are likewise known.
Binder mixtures for metallic base layers are known from DE-A-41 15 948 which consist of special polyurethane elastomers, polyester resins and amino resins and which optionally contain polymeric, crosslinked acrylate microgels. These mixtures of binders and acrylate microparticles may then be further processed to yield multilayer lacquer coatings. EP-A-0 379 158 describes thermoplastic coating compositions based on polyurethanes which are stabilised by means of ionic groups, together with up to 12% of acrylate resins as an added component. These are then used to coat plastic substrates.
DE-A-41 09 477 describes polyurethane dispersions based on polyester/isocyanate reaction products stabilised by ionic groups, which products are optionally chain-extended. After the addition of up to 80% of acrylate resins, reactive film adhesives may be produced therefrom.
All these known coating compositions based on mixtures of acrylate and polyurethane resins have the disadvantage that they occasion problems relating to storage stability. Moreover, there are often problems of intercoat adhesion if further subsequent layers are applied.
JP-A-04 103614 describes urethane/acrylic copolymers. These are produced by preparing a urethane prepolymer containing urea groups in the form of an aqueous dispersion. Acrylic monomers are polymerised in this aqueous dispersion. The resultant aqueous dispersion is reacted with an isocyanate prepolymer. Due to the manner of their production, the resultant copolymers are non-homogeneous and may lead to compatibility problems when used in coating compositions, which may also result in reduced storage stability and turbidity and in compatibility problems with other binder components, such as for example polyesters.
DE-A-26 63 307 describes the emulsion polymerisation of vinyl monomers in polyurethane dispersions. These dispersions are also used to produce coatings. They are produced by emulsion polymerisation in polyurethane dispersions, i.e. it is principally a mixture of pure acrylate polymers which is produced. Such polymer dispersions are also described in DE-A-37 22 005. They have the disadvantage that storage stability and intercoat adhesion problems occur here too. Furthermore, the necessary external emulsifiers lead to poor water resistance when used in multilayer lacquer coatings.